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The research has also helped to answer a problem that has baffled palaeontologists for decades.

THE TAIL OF ROMEO AND JULIET COULD HELP SEX DINOSAURS

A pair of turkey-sized ‘egg thief lizards’ dubbed Romeo and Juliet were found lying next to each other in a 75-million-year-old rock in the mid-90s, but it has taken until now for experts to determine the sex of the 'lovers'.

Now, researchers believe they have come up with a way to tell fossils of small feathered male dinosaurs from those of females.

They say that the key differences between the sexes lie in bones near the base of the tail.

Paleontologistsat the University of Alberta examined a pair of fossils found in Mongolia, which were first described in 2001.

One is a complete skeleton, while the other is missing the middle and end of its tail.

the researchers looked at indirect evidence of sexually dimorphic display structures, which are evident today in birds.

Examples include the fans of peacocks, the tall crests of roosters or the long tail feathers of some birds of paradise, which are used to attract and court mates.

They are almost always much larger in males who do the courting, than in females, who do the choosing.

The team theorised that like birds, small feathered dinosaurs such as oviraptors used long feathers on their tails to attract mates, rather than for flying.

Some stegosaurus individuals have been found with wide plates while others had tall plates with a sharper tip that were 45 per cent smaller.

Experts believed that these may have been two distinct species or individuals of a different age.

However, using modern scanning techniques on a group of stegosaurus discovered at a 'graveyard' in central Montana, scientists have unraveled the answer.

They found dinosaurs that had both sets of plates in the graveyard, suggesting they lived side-by-side.

Apart from the different plates, their skeletons were identical.

This makes it unlikely that they were different species and CT scanning has shown that the bones were not morphing into one or the other.

This suggests they belonged to two different sexes instead.

The research team believe that the males may have had the larger, broader plates for use in mating displays while the females had the sharper taller plates, that may have helped to discourage unwanted attention or predators.

Evan Saitta, who conducted the work at the University of Bristol, said: 'As males typically invest more in their ornamentation, the larger, wide plates likely came from males.

'These broad plates would have provided a great display surface to attract mates.

Stegosaurus, a herbivorous dinosaur, lived around 150 million years ago during the late Jurassic period in the western United States.

Along with its bony back, it also had large spikes on its tail that are thought to have been for defence or fights with rivals.

Stegosaurus (above) lived around 150 million years ago but scientists have been baffled by differences between the shape of the plates on some fossils - leading some to believe they were different species

The new research on the armoured plates may help scientists determine whether 'Sophie', the stegosaurus at the Natural History Museum in London pictured above, really is a female or if it was in fact a male dinosaur

Some dinosaur experts have speculated that the bony plates may have been covered in skin filled with blood vessels so they could flush with colour.

This could have acted as a warning to predator or being used as a way of attracting attention of the opposite sex.

Most dinosaur remains are difficult to determine the sex but there are a few that have been found to exhibit differences between males and females.

Some had large crests or nose horns that may have been sexual features.

Professor Paul Barrett, a palaeontologist at the Natural History Museum in London where one of the most complete stegosaur skeletons has just gone on display, said: 'It is interesting because it would add to the debate on what the plates were for.

'Sexual dimorphism is something we see a lot in modern animals, although it's not particularly prevalent in living reptiles or in living birds.'

However he added: 'I'd like to see a lot more information on the graveyard site itself, the different sizes of the specimens, how they were arranged in the ground, more detail on the geology, and on the analyses that were done.'

Evan Saitta (shown left with a stegosaurus armoured plate, used a CT scanner to examine the plates to see whether they might belong to different sexes. He is shown on the right cutting into the fossil to take samples